Wide area seabed analysis

ABSTRACT

A seabed region ( 18 ) that lies under a seabed surface area of over one square meter, is analyzed by comparing a core sample taken near the middle of the region and/or data from a geotechnical insitu cone penetrometer installed at the middle of the region, to an acoustic analysis of the region. Locations of the acoustic analysis are precisely correlated to the location of the core test sample or cone test by mounting an acoustic imaging apparatus ( 16 ) that holds acoustic transducers ( 44, 46 ), on a carriage ( 26 ) that is positioned on the core drill ( 12 ) or cone penetrometer barrel staff. The carriage of the acoustic imager apparatus is clamped to the core drill when the core drill is not rotating. An arm ( 30  and/or  32 ) is supported on the carriage through a frame ( 28 ), with at least one acoustic generator ( 44 ) and one acoustic echo detector ( 46 ) mounted on the arm. The arm can be rotated to positions lying about the drill axis ( 14 ) to accurately scan a wide area.

BACKGROUND OF THE INVENTION

There are many cases wherein it is necessary to determine the structureof the sea bed in a limited region. One case is where piles are to bedriven into the sea bed to support a construction project such ascolumns of an offshore hydrocarbon loading platform or of various typesof structures that are to lie offshore. If the seabed below a certaindepth is consolidated (firm and secure) then piles driven therein willremain stationary, while if piles are driven into soft subsea soil thenthe piles are not as secure and soil strength must be considered. Thereis also the costly refusal of the pile by the presence of boulders. Oneway to determine the condition of the sea bed is to produce samples ofthe sea bed using core drills, which range in diameter between about 5centimeters (2 inches) and 30 centimeters (12 inches). Another way is toconduct insitu cone tests. In insitu cone tests, a cone containingsensors is driven into the seabed and seabed characteristics at thatlocation are measured. These two methods will sometimes be collectivelyreferred to herein as seabed penetration measurement, by a seabed datapenetrator. Since offshore core drilling and insitu cone tests areexpensive and difficult to conduct, only a limited number of locationsare drilled or interrogated by a cone. This leads to uncertainty aboutthe condition of the sea bed. For example, if the core sample shows rockmaterial extending down from a predetermined depth, there is uncertaintyas to whether the rock is bedrock or is part of a boulder, or is part ofa discontinuous hard pan layer.

The seabed can take the form of soft sedimentary lenses, boulders and/orcobble stones, a glacial till (clay, sand, gravel, and bouldersintermingled), hard pan (compacted clay soil), mud layers, gas hydratesand gaseous sediments, and frozen soil. Many of these seabed materialsare of different conditions when lying in situ (in the sea bed) thanwhen present in a core sample, as where liquid and/or gas escape and/orvery fine particulates drop out of the core or the temperature changes.It is possible to analyze the seabed by acoustic (sonar and seismic)apparatus wherein the sound is directed at the sea bed and the echos aredetected. The echos indicate the reflectivity, attenuation,back-scatter, and velocities of sound at selected frequencies in thematerials, from which the characteristics of the sea bed can beestimated. The interpretation of such acoustic sea bed characteristicsis a more reliable presentation of the spatial extend of the layers thanfrom a core sample or insitu core test alone. Acoustic imaging can covera much wider area and at lower cost. It can also provide for lateralconfirmation of the physical core properties.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method andapparatus are provided for imaging a sea bed by imaging it, whichenables the more accurate evaluation of a region lying under a seabedsurface area of more than one square meter. The method includes the useof a core drill or insitu geotechnical cone to produce detailedinformation about a small volume of the sea floor. The same core drillor cone is used to support and accurately locate a wide area acousticimaging apparatus during its movement to obtain acoustic data about alarge area of the seabed. The acoustic apparatus includes a clampingcarriage that can slide down the shank of the core drill (or cone) andthen clamp to the core drill. An arm is supported on a frame that is, inturn, supported on the carriage. The arm extends radially away from thedrill and holds at least one set of transducers. These transducersinclude an acoustic generator that produces acoustic radiation and anacoustic detector that detects acoustic radiation that represents echosfrom the seabed.

With the core drill (or cone) lying adjacent to, or preferably againstthe sea floor, before or after a core sample has been drilled, theacoustic generator on the arm is operated to produce acoustic echos,with the output of the detector recorded. After acoustic readings havebeen taken at a plurality of locations along the arm, the carriage isrotated around the core drill as in increments of 15°, with acousticreadings taken at each angular position. As a result, a large and moreaccurate assessment of the sea bed is made, based on both the coresample and the acoustic imaging. A general assessment of the sea bedover the considerable area that has been acoustically imaged, is mademore definite by comparing the assessment at areas acoustically similarto where the core sample was taken, to the actual core sample.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a seabed evaluation apparatus of theinvention in an installation configuration, with the arms projectingupward from the clamping carriage.

FIG. 2 is a side elevation view of the apparatus of FIG. 1, with thearms deployed to imaging positions.

FIG. 3 is a view of a map of the seabed at a particular depth.

FIG. 4 is an isometric view showing the acoustic characteristics of aseabed at vertically spaced planes, and also showing a drill core.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a seabed evaluation complex 10 for evaluating a seabedregion 18, which includes a core drill 12 that extends along a primarilyvertical axis 14 and an acoustic interrogation apparatus, or acousticimaging apparatus 16. The core drill is of the usual type that includesa hollow drilling portion 20 and a shank 22. The shank is rotated orpounded down by Equipment (not shown) at the sea surface, while the coredrill is lowered into the seabed 24 and thereafter pulled up to thesurface so a core sample can be recovered. Instead of penetrating theseabed with a core drill, the seabed can be penetrated by an insituGeotechnical cone, which is a device with sensors that is pounded orotherwise inserted into the seabed. Both devices can be referred to as aseabed data penetrator, which penetrates deep (at least one meter) intothe seabed to gather data. The acoustic apparatus 16 of FIG. 1 includesa clamping carriage 26 that is centered on the drill axis and that canclamp (i.e. fix itself) to the drill. The apparatus also includes arotatable frame 28 that is rotatable about the carriage. The acousticapparatus also includes a pair of arms 30, 32 that are pivotallyconnected to the frame at primarily horizontal axes 34, 36 that arecircumferential to the drill axis.

FIG. 1. shows the acoustic imaging apparatus 16 during the course of itsinstallation to allow acoustic reading to be taken. The arms 30, 32 havebeen pivoted up so they extend primarily vertically and parallel to thedrill axis to facilitate sliding them down along the drill. With thecarriage lower end 38 lying against the sea bed surface 40 (FIG. 2), thearms are pivoted down to the configuration of FIG. 2. At least onetransducer set 42 which includes an acoustic generator 44 and anacoustic detector 46 is established at a particular position along eacharm 30, 32, such as at the radially outer end of each arm. Applicantnotes that FIG. 2 shows two modes for the transmission and reception ofacoustic energy. One mode is the use of multiple acoustic detectors 46whose positions (e.g. fixed on the carriage 26) do not change and withapplicant repeatedly repositioning only the acoustic generators 44. Inthe other mode, co-located transmitter(s) and receptor(s) move along thearm.

With the acoustic generator 44 at a selected position on the arm, theacoustic generator is energized by electronic signals such as shortpulses, to produce acoustic waves that penetrate into the seabed 24. Topenetrate to a depth of up to 10's of meters, applicant producesacoustic waves of a frequency that is typically 1 KHz to 50 KHz. Theacoustic waves generate echos which are detected by the detectors 46.The times after acoustic transmission when various parts of an echo aredetected and the amplitudes of the detected echo parts indicate manycharacteristics of the seabed. These include the density at variousdepths (which can indicate rock or soil), and the locations of the topand bottom of boulders (where there are strong reflections) and othermaterials in the sea bed. The acoustic generator produces a beam thatinterrogates (produces images of) a tall column-shaped location underthe seabed surface.

Each acoustic generator is repeatedly moved along an arm 30, 32 to imagemany column-like volumes spaced along the arm. After all locations alongone arm position are interrogated, the arm is pivoted, as by 15°, aboutthe drill axis 14. At each rotational position, the seabed is insonified(echos are detected from transmitted sonic pulses) at a plurality ofpositions of the acoustic generator along each arm. The result is atwo-dimensional map such as shown at 50 in FIG. 3 for each of aplurality of depths under the seabed surface. Each arm 30, 32 (FIG. 2)has a length of more than 0.5 meter, so the area of the seabed surfaceunder which the seabed is insonified, is more than one square meter(more than 10 square feet). Preferably, each arm has a length of aplurality of meters, so the area of the seabed under which the seabed isacoustically examined is a plurality of 10's of square meters.

In one example, the drill core indicates rock at location 54, while themap 50 of FIG. 3 indicates rock at 56 which form boulders because theyhave small horizontal dimensions. The map 50 indicates a wide expanse ofrock at 58 which could be bedrock and which could be furtherinterrogated. Sometimes even the core sample is deceiving as where itcontains material that changes state under pressure or contains fluidsor has been blocked by large particulates during the sampling. FIG. 4shows the physical core 52 which extends about 20 meters below theseabed surface 40, and shows four acoustic images taken at 5 meterintervals. The acoustic images indicate fine sand at 51, course sand at53, boulders at 56 and fine clay at 55.

It is important that the positions of locations on the acousticexamination map 50 be accurately correlated to the position of the coresample(s) at 52 for that volume of the sub-seabed. The correlationshould be within an inch (2.5 centimeters) in perpendicular lateraldirections, and also be accurate in a vertical direction. The acousticimaging apparatus 16 shown in the figures enables such close correlationof positions.

After applicant lowers the drill (FIG. 1) to the seabed (and usuallyafter a core is drilled), applicant positions a passage 29 in thecarriage 26 so it receives the drill 12. Then, applicant lowers thecarriage along the drill until the carriage lower end 38 lies againstthe seabed 24 as in FIG. 2. An umbilical 62 extends from a facility atthe sea surface down to the carriage. The umbilical is used to lower thecarriage until the carriage lower end contacts the sea floor. A cable 64also extends to the sea surface. A clamp 65 on the carriage is thenoperated to clamp the carriage to the drill, at the drill shank. Thedrill 12 preferably lies in contact with the seabed at the walls of acore hole 66 that has been drilled or that is to be drilled, toaccurately position the acoustic apparatus with respect to the corehole. With the carriage fully lowered, a winch 68 is operated to lowerthe arms 30, 32 until they are horizontal, as shown in FIG. 2. Applicantcan use a single arm 30, or can use two arms to interrogate morerapidly.

With the arms lowered, the acoustic generators 44 are energized and theechos are detected by the receivers 46. After each acousticinsonification by detecting the echos, the transducer(s) is moved alongthe arm 30, 32 to a new position. The column-shaped volumes imaged bythe transducers 44, 46 usually overlap. After sounding a series ofvolumes lying under the length of the arm, the arm 30, 32 is rotated toa new position. Data from the interrogation apparatus is stored in adata file 48 although it can be transmitted to a recorder at the seasurface. An actuator apparatus typically formed by an electric motor 70with gear set 72 or pneumatic or hydraulic actuator, rotates the frame28 on which the arms 30, 32 are mounted, about the carriage 26 that is,clamped to the drill. Each rotation angle is preferably about 15° andproceeds in typically twelve to twenty-four steps to provide twenty-fourangularly spaced arm positions for the two arms. However, if an area ofspecial interest is found (e.g. 58 in FIG. 3) the frame may be rotatedin steps of perhaps 1°.

In a system that applicant has designed, the arms 30, 32 each had alength of 7 meters. As a result, a volume of the sea bed wasacoustically interrogated which lay under a sea floor area of 68 squaremeters. The arms were located above the seabed by a distance A of morethan a Meter, and actually was about 3 meters above the sea floor, whichallowed the pulse initially generated by the generator 44 to producesound waves of a frequency of 1 KHz to 20 KHz in a broadening beam thatpassed into the seabed.

The ability to precisely position the transducers 42, enables applicantto employ synthetic aperture sonic techniques to augment the analysis ofthe seabed. In synthetic aperture sonic techniques, applicant, detectsand co-locates the phases of returned (reflected and/or refracted)signals, or echos, in addition to their amplitude and time of detection(after transmittal), which enables a more precise analysis of seabedcharacteristics.

Thus, the invention provides a method and apparatus for analyzing aseabed volume that lies under an area of more than one square meter ofthe sea bed surface. The invention involves the penetration of over onemeter of the seabed by a seabed data penetrator, and the acousticimaging, or interrogation, of a volume in the sea bed that lies aroundthe location where the seabed penetration was made to gather data from ahole in the seabed. This allows the evaluation of a large volume of thesea bed, using only one or a limited number of core drillings and/orinsitu cones. This is accomplished by using the acoustic interrogationto evaluate the lateral extent of layers in the seabed and bycross-correlating with the core sample and/or data from the cone tocheck that acoustic iterations between the two sources of informationproduce a final consistent calibrated interpretation of conditions ofthe seabed. Accurate information about the location of the core samplewith respect to the locations where the acoustic evaluation data weretaken, is assured by positioning the acoustic transducer(s) on anapparatus that is mounted on the core drill, with the bottom of thecarriage placed in contact with the seabed while the core drill lies incontact with the sea bed at the location where the core was taken or isto be taken. This is accomplished by mounting the transducers on anarm(s) that rotates about the axis of the drill.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art, and consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

1. A method for interrogating a region of a sea bed which includesforcing a seabed data penetrator into a hole at a penetration locationin the sea bed and examining the condition of the seabed as indicated bysaid seabed data penetration, including: using the seabed datapenetrator as a positioner to locate an acoustic transducer device withrespect to said hole of said penetration location, and using saidacoustic transducer device to direct acoustic energy at the sea floorand detect echos thereof, including recording echos from a seabed regionthat lies under a seabed surface having an area that is more than tentimes the cross-section of said hole of said penetration location thatwas penetrated by said data penetrator; comparing the properties of thesea bed at selected depths as indicated by said seabed data penetrationand as indicated by said recorded echos.
 2. The method described inclaim 1 wherein: said acoustic transducer device includes a carriagethat is fixable to said core drill and a frame that is moveably mountedon said carriage to move to different horizontally-spaced locations andthat carries a plurality of acoustic transducers; said step of directingacoustic energy includes moving said frame to locate said acoustictransducers at each of a plurality of horizontally spaced locations overthe sea floor.
 3. The method described in claim 2 wherein: said seabeddata penetrator has a primarily vertical axis, said carriage is mountedon the seabed data penetrator, and said frame is pivotally mounted onsaid carriage to pivot about said axis of said seabed data penetrator,and said step of moving said frame includes pivoting said frame aboutthe axis of said seabed data penetrator.
 4. The method described inclaim 1 wherein: said step of using said acoustic transducer deviceincludes directing energy at the sea floor and detecting echos, whilethe seabed data penetrator lies in contact with the sea bed but is notmoving.
 5. The method described in claim 1 wherein: said seabed datapenetrator is a core drill and said acoustic transducer device includesa carriage that is fixable to said core drill, said carriage having alower end, and said step of using the seabed data penetrator as apositioner includes lowering the carriage along said core drill untilsaid lower end of the carriage contacts the sea floor.
 6. The methoddescribed in claim 1 wherein: said step of recording echos includesrecording the relative phases of echos that originate from adjacentareas of the seabed, to obtain a detailed analysis of a region thatincludes said adjacent areas.
 7. A method for determining thecharacteristics of an area under a seabed surface, comprising:acoustically imaging a volume of the sea bed that lies below said seabedsurface, where said area is a plurality of square meters, by directingacoustic energy at the sea floor and detecting echos using acoustictransducers, at a plurality of locations above said seabed surface area;lowering a seabed data penetrator into the sea floor, along apenetration axis, to produce data indicating the condition of the seabedby penetration thereof at a location in the seabed that lies under saidseabed surface area and maintaining said seabed data penetrator incontact with the seabed while conducting said step of acousticallyimaging; said step of directing acoustic energy and detecting echos at aplurality of locations, includes positioning and repositioning saidacoustic transducers by actuator apparatus that is mounted on saidseabed data penetrator, whereby to use the seabed data penetrator as areference.
 8. The method described in claim 7 wherein: said step ofpositioning and repositioning said acoustic transducers includespivoting an arm on which said acoustic transducers are mounted, aboutsaid penetration axis.
 9. The method described in claim 8 wherein: saidstep of pivoting an arm about said penetration axis includes mountingsaid arm on a frame that is rotatable on a carriage that is supported onsaid seabed data penetrator, and operating a motor to, turn said frameto different positions about said penetration axis.
 10. The methoddescribed in claim 7 including: lowering said seabed data penetratordownward to the sea floor, supporting said acoustic transducers on anarm that is pivotally supported by a carriage on said seabed datapenetrator, receiving a shank of said penetrator on said carriage andlowering said carriage along said shank.
 11. Apparatus for interrogatinga seabed which includes a seabed data penetrator that produces data onthe condition of the seabed from penetration of the seabed, comprising:a carriage that has a carriage passage for receiving a part of saidseabed data penetrator and a clamp for clamping the carriage to thepenetrator, a frame that carries acoustic transducers, and a mechanismthat moves said frame on said carriage to move said acoustic transducersover each of a plurality of locations over the seabed with respect tosaid passage and therefore with respect to said penetrator.
 12. Theapparatus described in claim 11 wherein: said frame includes anelongated arm, at least one of said transducers is moveable to differentlocations on said frame, and said frame is rotatable in steps about saidcarriage.
 13. The apparatus described in claim 11 wherein: said carriagehas a lower end for directly contacting the seabed and that extends morethan a meter below said acoustic transducers.
 14. A method forinterrogating a region of a seabed which includes rotating and loweringa core drill into a location in the seabed and examining the resultingcore sample, including: using the core drill as a positioner, to locatean acoustic transducer device with respect to said core sample, andusing said acoustic transducer device to direct acoustic energy at thesea floor and detect echos thereof, including recording echos in aseabed region that lies under a seabed surface more than ten times thecross-section of said core sample, that includes said location wheresaid core sample was taken; comparing the properties of the sea bed atselected depths as indicated by said core sample and as indicated bysaid recorded echos.
 15. The method described in claim 14 wherein: saidacoustic transducer device includes a carriage that is verticallyslideable along and fixable to said core drill and a frame that ismoveably mounted on said carriage and that carries a plurality ofacoustic transducers to move said transducers to differenthorizontally-spaced locations; said step of directing acoustic energyincludes moving said frame to locate said acoustic transducers at eachof a plurality of, horizontally spaced locations over the sea floor anddirecting energy at each of said locations.
 16. The method described inclaim 15 wherein: said core drill has a primarily vertical axis, saidcarriage is mounted on the core drill, and said frame is pivotallymounted on said carriage to pivot about said axis of said core drill,and said step of moving said frame includes pivoting said frame aboutthe axis of said core drill.